Swift - 从 UICollectionViewCell 内的视图中删除约束

Question

我以编程方式在自定义 UICollectionViewCell 内的视图中添加了一些约束，但我注意到当一个单元格被重用时，可能会重新添加约束。

这是正在记录的内容：

> Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints
> to catch this in the debugger. The methods in the
> UIConstraintBasedLayoutDebugging category on UIView listed in
> <UIKitCore/UIView.h> may also be helpful. 2020-07-21
> 01:00:10.466637+0300 AutoLayout Message Cells[5227:336361]
> [LayoutConstraints] Unable to simultaneously satisfy constraints.
>   Probably at least one of the constraints in the following list is one
> you don't want.   Try this:       (1) look at each constraint and try to
> figure out which you don't expect;        (2) find the code that added the
> unwanted constraint or constraints and fix it.  (
>     "<NSLayoutConstraint:0x600000435090 AutoLayout_Message_Cells.MessageCollectionViewCell:0x7fbd76e24670.height
> == UIView:0x7fbd76e291b0.height   (active)>",
>     "<NSLayoutConstraint:0x6000004350e0 V:|-(8)-[UILabel:0x7fbd76e29510'UOA']   (active, names:
> '|':UIView:0x7fbd76e291b0 )>",
>     "<NSLayoutConstraint:0x6000004351d0 UILabel:0x7fbd76e29510'UOA'.height == 30   (active)>",
>     "<NSLayoutConstraint:0x600000435220 V:[UILabel:0x7fbd76e29510'UOA']-(-12)-[UILabel:0x7fbd76e29790'2h']  
> (active)>",
>     "<NSLayoutConstraint:0x600000435310 UILabel:0x7fbd76e29790'2h'.height == 30   (active)>",
>     "<NSLayoutConstraint:0x6000004353b0 V:[UILabel:0x7fbd76e29790'2h']-(16)-[UITextView:0x7fbd7805b200'Lorem
> ipsum dolor sit ame...']   (active)>",
>     "<NSLayoutConstraint:0x600000435400 UITextView:0x7fbd7805b200'Lorem ipsum dolor sit ame...'.bottom ==
> UIView:0x7fbd76e291b0.bottom - 1   (active)>",
>     "<NSLayoutConstraint:0x60000042a990 'UIView-Encapsulated-Layout-Height'
> AutoLayout_Message_Cells.MessageCollectionViewCell:0x7fbd76e24670.height
> == 50   (active)>" )

这是我手机的代码：

import UIKit

class MessageCollectionViewCell: UICollectionViewCell {
    @IBOutlet var textView: UITextView!
    @IBOutlet var schoolLabel: UILabel!
    @IBOutlet var dateLabel: UILabel!
    
    override func awakeFromNib() {
        super.awakeFromNib()
        setupConstraints()
    }
    
    fileprivate func setupConstraints() {
        setupCellConstraints()
        setupSchoolLabelConstraints()
        setupDateLabelConstraints()
        setupTextViewConstraints()
    }
    
    fileprivate func setupCellConstraints() {
        self.contentView.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            self.contentView.widthAnchor.constraint(equalToConstant: UIScreen.main.bounds.width - (self.frame.origin.x * 2))
        ])
        
        NSLayoutConstraint.activate([
            self.widthAnchor.constraint(equalTo: self.contentView.widthAnchor),
            self.heightAnchor.constraint(equalTo: self.contentView.heightAnchor)
        ])
    }
    
    fileprivate func setupSchoolLabelConstraints() {
        schoolLabel.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            schoolLabel.topAnchor.constraint(equalTo: self.contentView.topAnchor, constant: 8),
            schoolLabel.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor, constant: 8),
            schoolLabel.trailingAnchor.constraint(equalTo: self.contentView.centerXAnchor, constant: 60),
            schoolLabel.heightAnchor.constraint(equalToConstant: 30)
        ])
    }
    
    fileprivate func setupDateLabelConstraints() {
        dateLabel.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            dateLabel.topAnchor.constraint(equalTo: schoolLabel.bottomAnchor, constant: -12),
            dateLabel.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor, constant: 8),
            dateLabel.trailingAnchor.constraint(equalTo: self.contentView.centerXAnchor, constant: 60),
            dateLabel.heightAnchor.constraint(equalToConstant: 30)
        ])
    }
    
    fileprivate func setupTextViewConstraints() {
        textView.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            textView.topAnchor.constraint(equalTo: dateLabel.bottomAnchor, constant: 16),
            textView.bottomAnchor.constraint(equalTo: self.contentView.bottomAnchor, constant: -1),
            textView.leadingAnchor.constraint(equalTo: self.contentView.leadingAnchor, constant: 8),
            textView.trailingAnchor.constraint(lessThanOrEqualTo: self.contentView.centerXAnchor, constant: 60)
        ])
    }

有没有办法在重新添加之前清除所有约束？

Answer 1

我也以编程方式而不是在情节提要中使用约束。您遇到的问题是某些约束不能共存。

例如在代码中我没有看到：

>     "<NSLayoutConstraint:0x60000042a990 'UIView-Encapsulated-Layout-Height'
> AutoLayout_Message_Cells.MessageCollectionViewCell:0x7fbd76e24670.height
> == 50   (active)>" )

某处（可能在故事板中？）您给出的高度为 50，但是：

单元格与 contentView 的高度相同：

self.heightAnchor.constraint(equalTo: self.contentView.heightAnchor)

在您提供的单元格中：

        schoolLabel.topAnchor.constraint(equalTo: self.contentView.topAnchor, constant: 8),
....
        schoolLabel.heightAnchor.constraint(equalToConstant: 30)

所以我们有 8 个形式的内容顶部 + 30 个高度 (38)，那么：

        dateLabel.topAnchor.constraint(equalTo: schoolLabel.bottomAnchor, constant: -12),
....
        dateLabel.heightAnchor.constraint(equalToConstant: 30)

您返回 12，然后再次返回 30 (38 - 12 + 30 = 56) 的高度。 我们超出了 50 个单元格，再加上您添加的文本：

        textView.topAnchor.constraint(equalTo: dateLabel.bottomAnchor, constant: 16),
        textView.bottomAnchor.constraint(equalTo: self.contentView.bottomAnchor, constant: -1)

其高度将导致负数（textView 顶部为 56 + 16 = 72，但底部为 50（我们看不到的约束）- 1 = 49 --> 它的高度为 -23